Have a pitcher plant in the conservatory - the worst affected room, but
have
never noticed a hoverfly or any other fly for that matter in any of the
pitchers.

What type of pitcher plant do you have? Some are better than others, I
recently had problems with wasps in the house, and a Sarracenia x Excellens
(a hybrid between S. minor and S. leucophylla) caught every wasp that went
near the window it was beside!

Regards,

Stuart Forbes
Edinburgh, Scotland